It is known to monitor the physical characteristics of structures and bodies using sensors. One such application is the monitoring of oil wells to extract such information as temperature, pressure, fluid flow, seismic, and other physical characteristics. The monitoring of oil wells presents certain challenges for conventional sensors because they must be placed in harsh environments (e.g., high pressures and temperatures). Historically, such monitoring has been dominated by the use of electronic sensors and optical sensors to a lesser degree.
Such conventional electrical sensors are limited for several reasons. The on-board electronics of such sensors must operate in a very hostile environment, which includes high temperature, high vibration, and high levels of external hydrostatic pressure. Such electrical sensors also must be extremely reliable, since early failure entails very time consuming and expensive well intervention. Electronics, with its inherent complexity, are prone to many different modes of failure. Such failures have traditionally caused less than acceptable levels of reliability when these electrical sensors are used to monitor oil wells.
There are numerous other problems associated with the transmission of electrical signals within well bores. In general, it is difficult to provide an insulated electrical conductor for transmitting electrical signals within well bores. Such electrical conductors are extremely difficult to seal against exposure to well bore fluids, which are at high temperatures, high pressures, and present a very corrosive environment. Such electrical conductors, once damaged by the fluids that penetrate the insulating materials around the electrical conductors, will typically short electrical signals. Additionally, electrical transmissions are subject to electrical noises present in some production operations.
It is typical to use an accelerometer to measure downhole seismic disturbances to determine the acoustic wave characteristics of underground layers in the proximity of the well bore. An accelerometer is generally a mass-spring transducer housed in a sensor case. The sensor case is coupled to a moving body, the earth, whose motion is inferred from the relative motion between the mass and the sensor case. Such accelerometers relate the relative displacement of the mass with the acceleration of the case, and therefore the earth in the proximity of the well bore. An array of accelerometers is typically placed along the length of a well bore to determine a time dependent seismic profile.
One prior art accelerometer is a piezoelectric based electronic accelerometer. The piezoelectric based electronic accelerometer typically suffers from the above-referenced problems common to electrically based sensors. In particular, most high performance piezoelectric accelerometers require power at the sensor head. Also, multiplexing of a large number of such sensors is not only cumbersome but tends to occur at a significant increase in weight and volume of an accelerometer array, as well as a decrease in reliability. Also, piezoelectric accelerometers operate poorly at the lowest frequencies in the seismic band.
It is also known to use optical interferometer accelerometers to measure the acceleration of certain structures, and that they can be designed with fairly high responsivities and reasonably low threshold detection limits. Some prior art types of fiber optic accelerometers include interferometric fiber optic accelerometers based on linear and nonlinear transduction mechanisms, circular flexible disks, rubber mandrels, and liquid-filled-mandrels. Some of these fiber optic accelerometers have displayed very high acceleration sensitivity (up to 104 radians/g), but tend to utilize a sensor design that is impractical for many applications.
For instance, sensors with very high acceleration sensitivity typically often have a seismic mass greater than 500 grams. This seriously limits the frequency range in which the device may be operated as an accelerometer. The devices are so bulky that their weight and size renders them useless in many applications. Other fiber optic accelerometers suffer either from high cross-axis sensitivity or low resonant frequency, or require an ac dither signal, and tend to be bulky (>10 kg), expensive, and require extensive wiring and electronics. Even optical interferometers designed of special materials or construction are subject to inaccuracies because of the harsh borehole environment and the very tight tolerances present in such precision equipment.
For many applications, the fiber optic sensor is expected to have a flat frequency response up to several kHz (i.e., the device must have high resonant frequency) and high sensitivity. For many applications, the fiber optic sensor must be immune to extraneous measurands (e.g., dynamic pressure) and must have a small foot print and packaged volume that is easily configured in an array (i.e., easy multiplexing).